Described herein are methods for making a silylamine, more specifically trisilylamine. Trisilylamine ((SiH3)3N or “TSA”) is a precursor that is finding use in the deposition of high purity silicon oxide films for gap fill applications in the semiconductor industry. Trisilylamine is a reactive precursor that does not require direct plasma excitation for film growth.
Stock, A.; Somieski, C. Ber. Dtsch. Chem. Ges. 1921, 54, 740 reported synthesis of TSA by a reaction of monochlorosilane (MCS) with ammonia (NH3) as shown by Equation (1):3SiH3Cl+4NH3→(SiH3)3N+3NH4Cl  (1)The above reference reported principal formation of DSA [(SiH3)2NH] in excess ammonia. DSA also was shown to decompose to SiH4 and polymeric silicon compounds or polysilazanes as shown by Equation (2):(SiH3)2NH→SiH4+[SiH2(NH)]x  (2)
In the presence of excess ammonia, polysilazanes, SiH4, DSA are the likely products. TSA and polysilazanes react violently with water to make SiO2, H2 and NH3. Depending on the polymer chain, the polysilazanes can be a volatile liquid to solid. Their vapor pressure is lower than TSA.
US Publ. No. 2011/0136347 (“the '347 publication”) describes a process for the production and delivery of a reaction precursor containing one or more silylamines such as TSA near a point of use. The process described in the '347 publication is conducted in gas and/or liquid phase at a temperature ranging from about −80° C. to about room temperature. The '347 publication further teaches adding an inert gas or using an organic solvent in the reaction vessel to reduce the formation of oligomers in the form (SiH2NH)n.
U.S. Pat. No. 8,409,513 (“the '513 Patent”) describes a tubular flow reactor and a process for the synthesis of silylamines such as TSA. According to the '513 patent, the reactor has a unique combination of characteristics found in plug flow and laminar flow devices. The reaction of ammonia with molar excess MCS was carried out at low pressure in gas phase.
The reference entitled “The Preparation and Some Properties of DiSilylamine”, B. J. Aylett, et al., Inorganic Chemistry, Vol. 5(1), p. 167 (1966) reported that DSA did not convert to TSA in gas phase even at 150° C. However, at 0° C., DSA was shown to convert to TSA. At 0° C., DSA will condense to a liquid.
U.S. Publ. No. 2013/0089487 (“the '487 publication”) describes a condensed phase batch process for synthesis of TSA. The process of the '487 publication incorporates a solvent such as anisole to act as a heat-transfer medium in which the ammonium chloride salt is dispersed and downstream product removal is devoid of salt formation.
To obtain high purity trisilylamine, it is important to minimize the formation of DSA, use excess MCS to complete the reaction to TSA, and minimize formation of polysilazanes from decomposition of DSA. To increase productivity and production volume, intermediate formation should be minimized during synthesis to speed the process. TSA synthesis via reaction (1) results in significant formation of ammonium chloride. For 1 unit weight of TSA, 1.5 unit weight of ammonium chloride is formed. Any polysilazanes formed will be heavies and can remain part of ammonium chloride matrix. Safe handling of the solid ammonium chloride requires minimization of polysilazane formation, safe and efficient method to remove polysilazanes from ammonium chloride prior to ammonium chloride disposal. Crude TSA is further purified to obtain high purity TSA as needed by customers.